An Efficient Compositional Model for Unconventional Reservoirs: Eagle Ford Formation Case Study

Abstract

The main complexity of the multi-component, multi-phase simulation in unconventional reservoirs arises because of the thermodynamic phase behavior and component transport in such small pores. To manage these issues, a computationally cost-effective implicit model is needed. This paper presents a new multi-component, multi-phase, dual-porosity numerical model including molecular diffusion for simulating fluid flow for the field applications in Eagle Ford formation, Texas. The model, which is based on the mass transport equations for each component, solves for the pressures and overall compositions simultaneously, then it solves for crossing phase boundaries and saturations sequentially. This new model, which determines the effect of molecular diffusion on production, is computationally robust and efficient. The vapor-liquid equilibrium calculation is performed using Peng-Robinson equation of state including the phase shifts and the capillary pressure effects on phase behavior. The rock and fluid data from Eagle Ford wells are used as an input for the simulation model to evaluate the accuracy and computational efficacy compared to the classic volume balance technique for unconventional reservoirs. The compositional mass transport equations can be solved using several variants of the fully implicit method in addition to a less stringent sequential volume balance technique. The latter is of interest because one can generate a simple compositional pressure diffusivity equation and sequentially solve for the composition. However, the volume balance technique is sensitive to the grid and time-step size. We compared our results with the volume balance method to examine the accuracy of both solution techniques. The degree of implicitness in our model not only provides better accuracy for the simulation but also eliminates the material balance errors occurring in explicit solutions. Therefore, our method provides a better understanding of the physical phenomena of fluid flow processes at the matrix-fracture interface in unconventional reservoirs.